It is well-known that particles travelling through a small orifice can be characterised with respect to size, concentration and conductivity by the use of an electrical impedance technique, widely known as the Coulter sizing (see V. Kachel, “Electrical Resistance Pulse Sizing Coulter Sizing”, Flow Cytometry and Sorting, Second Edition, pp. 45-80, 1990 Wiley-Liss).
Counting and sizing of particles by the impedance principle is an internationally approved method that is being used in most haematology-analysers and particle counting equipment. The method is based on measurable changes in the electrical impedance produced by non-conductive particles in an electrolyte. A small opening, called the “aperture” or “orifice”, connects two electrically isolated chambers, where electrodes have been provided to contact the electrolyte. The orifice applies a restriction to the electrical path, whereby a sensing zone is established through which the particles are aspirated. In the sensing zone each particle will give rise to a displacement of the surrounding electrolyte, thus blocking part of the current-path and giving rise to a voltage pulse. By this method several thousand particles per second can be characterised with high precision.
It is also well-known that the peak amplitude of the voltage pulses generated by the particles are closely correlated to the size of the particles, and therefore it is desirable to be able to determine the peak amplitude of voltage pulses in a simple and reliable way and at a low cost.
The haematology analysers are used for counting and differentiating blood cells such as thrombocytes (blood platelets), leucocytes (white blood cells) and Erythrocytes (Red blood cells). The white blood cells can be further characterised by size into the three sub-populations lymphocytes, monocytes and granulocytes.
Typically, known haematology analysers constitute a complete and self-contained apparatus for sequential testing of a large number of blood samples. Such analysers have a relatively complicated flow system with containers for chemicals and rinsing liquids. The flow system is automatically subjected to a rinsing cycle between sample measurements. A calibration sample, e.g. containing polystyrene particles with known diameters may be aspirated into the flow system at regular intervals for calibration of the analyser.
In WO 01/11338, an alternative haematology analyser is disclosed comprising a disposable cartridge and a docking station. The cartridge contains the flow system including the orifice and electrodes for Coulter counting. The docking station interfaces to the flow system and the electrodes and controls the measurement cycle. Blood is sampled by the cartridge, which is then inserted into the docking station for determination of the number of blood cells in the blood sample. Since the cartridge is removed and discarded after testing, the docking station does not need a rinsing system for rinsing of the station between testing of different blood samples whereby a simple and easy to use haematology analyser is provided. This makes it possible to perform measurements near the patient without participation of specialized personnel allowing convenient, accurate monitoring of the state of the patient with a very small delay between sample taking and measurement result.